I started getting the "proficiency too low" message recently, and I'm sure I never got it before, with the same build and equipment.
I have 6 PT, 120 throwing wpf, open sallet with coif, rus scale armor, hourglass gauntlets and mail chausses and the web calculator tells me this :
Armor Weight: 23.3
Effective Armor Weight: 22.9
Effective WPF: 32
WPF penalty from weight: 10
WPF penalty from PT/PD: 78
So I should have an excess 32 wpf, yet I get the message ingame. It started yesterday however I was inactive for about 2 weeks before. It was also the first time I played with WSE 2.